The present invention relates generally to ultrasound imaging systems of the sector scan type, and in particular to a scan converter for the sector scan type ultrasound imaging system for converting echo signals generated in the sector scan field into signals for raster scan.
In the sector scan type ultrasound imaging system, piezoelectric transducers of a linear array are energized by successively delayed bursts of high frequency pulses so that the transmitted ultrasound energy forms a main ultrasound beam deflected at an angle depending on the delay intervals of the transducer array. The delay intervals are controlled so that the main beam is deflected at various angles with respect to the perpendicular to the transducer array to form a sector scan field. Ultrasound echos reflected from targets arrive at different times at the transducers. The echo signals respectively received by the transducers are delayed so that the combined echo signal can be regarded as having returned from a sharply defined target point.
The main transmitted ultrasound beam is conventionally deflected by equal angular increments. In order to prevent Moire fringes from appearing in the raster display field, the received echo signals are sampled at such intervals that the sampled points in the sector scan field exactly correspond to the picture elements or dots in the raster scan field. The sampled echo is stored successively in the storage cells of a matrix array in the direction of its columns and then retrieved in the direction of its rows. Due to the equal incremental angular deflection, however, the spacing between the sampled points corresponding to the picture elements successively arranged in each raster scan line varies in proportion to the tangent of each deflection angle with respect to the perpendicular to the transducer array. It is therefore necessary that the stored echo data be retrieved at varying speeds, and this would require the provision of a read only memory having a substantial capacity sufficient to accommodate address data for writing the echo data into and reading it from the random access memory.